I. Field of the Invention
This invention relates to color change devices incorporating thin anodic films capable of generating colors by light interference effects, to methods of making such devices, and to closures and other articles incorporating such devices.
By the term "color change device" we mean a laminated structure which exhibits a first color over a suitably large surface area but which is capable of exhibiting a noticeably different second color over the whole or a part of the surface area when the structure is physically disturbed in some way, e.g. when the constituent layers are peeled apart or when the device is punctured or cut.
II. Description of the Prior Art
There is currently a growing need for color change devices incorporating structures which undergo some kind of irreversible and readily-observable change when the constituent structures are peeled apart or otherwise disturbed. Such devices may be incorporated, for example, into the closures of containers or packages in such a way that an irreversible visible change is observable when the containers or packages are opened. Alternatively, when identity documents or cards are laminated for security, devices of the above type may be incorporated into their structures to warn of tampering. Furthermore, there is a growing market for "instant win" type lottery tickets which contain a message concealed beneath a peelable or scratchable obscuring layer and it would be advantageous to incorporate color change devices into such tickets to discourage unauthorized viewing of the message prior to sale.
Various types of structures which undergo irreversible visual changes are already known. For example, U.S. Pat. No. 4,557,505 issued on Dec. 10, 1985 to Richard M. Schaefer, et al discloses a transparent tape which becomes opaque when subjected to stress, e.g. when peeling or tearing of the tape is attempted, and similar "stress whitening" properties of plastics materials are utilized in the devices of U.S. Pat. No 4,489,841 issued on Dec. 25, 1984 to Mortimer S. Thompson and U.S. Pat. No. 4,448,317 issued on May 15, 1984 to Mortimer S. Thompson. Another approach to the problem has been the use of micro-encapsulated dyes which change color upon exposure to air when the capsules are ruptured (e.g. U.S. Pat. No. 4,519,515 issued on May 28, 1985 to Milton Schonberger; U.S. Pat. No. 4,480,760 issued on Nov. 6, 1984 to Milton Schonberger; and U.S. Pat. No. 4,424,911 issued on Jan. 10, 1984 to Joseph A. Resnick). Additionally, much attention has recently been directed to the use of holograms having a three dimensional visual effect, and iridescent optical multilayer films, made by vacuum deposition, which exhibit a distinctive color change with viewing angle, such effects being easily destroyed when the structures are damaged.
The disadvantages of the known devices are that they are either expensive to produce (e.g. the holograms and optical multilayer films), release contaminating chemicals (e.g. microencapsulated dyes) or can be defeated or replaced if sufficient care is taken (e.g. the stress-whitening plastics).
We have previously found that color change devices can be made from a peelable laminate that relies on direct and intimate contact between at least two layers to generate an intense non-dichroic interference color. When the layers are peeled apart, the generated color disappears (or changes to a different color depending on the structure of the device) and is difficult or impossible to regenerate because the required direct and intimate contact cannot be restored by normal means (e.g. pressing or gluing the layers back together again). In a preferred form, the device of this kind comprises a thin layer of a so-called "valve" metal (e.g. Ta, Nb, Ti, Zr and Hf) having a very thin non-porous overlying anodic layer of the valve metal oxide. Such a structure generates an intense color by a light interference and absorption effect (i.e. interference takes place between light reflected at the metal and oxide surfaces and some light absorption takes place at the metal-oxide interface). Normally, anodized oxide layers adhere strongly to the underlying metal, but we have found that the presence of certain adhesion-reducing agents (e.g. fluoride ions) during anodization reduce this adhesion in a uniform and reliable way and hence make the structure peelable at the metal-oxide interface. This invention is the subject of our prior U.S. Pat. No. 4,837,061 issued on June 6, 1989 (the disclosure of which is incorporated herein by reference).
Tamper-evident structures of the above kind undergo a substantially irreversible color change when the two adjacent layers are separated from each other because the direct and intimate contact required for color generation is difficult or impossible to restore once the adjacent layers have been peeled apart, and the substantially irreversible color change acts as evidence that the layers have been separated and consequently that the structure has been disturbed. Since the color change is based on a light interference and absorption phenomenon, which is a physical rather than a chemical phenomenon, the operability of the structure is substantially unaffected by heat, humidity, aging etc.
While tamper-evident devices of the above kind are extremely effective and useful, they suffer from the disadvantage that the materials capable of generating the desired intense colors are inconvenient to anodize as they require high voltages (i.e. anodizing to high voltage at constant current). Our prior devices also require the use of quite large amounts of expensive materials, such as tantalum. Moreover, only very thin oxide layers can be produced and this limits the colors that can be generated, generally precluding dichroic films which would be realized by thicker layers. A dichroic film is one which exhibits a particular color when viewed from one angle, say at normal incidence, but a different color when viewed from another angle. In general, for such films the color changes continuously through several hues as the viewing angle is varied and the films have accordingly also been called optically variable films. Dichroism is a desirable feature for some applications due to consumer appeal. Also, since the dichroic feature can not be reproduced by color photocopiers, it confers an additional element of security to a color change device used in tamper evident structures. In some cases though, dichroism is to be avoided since it may confuse the consumer as to which color change is to be taken as evidence of tampering, and so it would be desirable to have the option of making the color change device either dichroic or non-dichroic.